(1) Field of the Invention
The present invention relates to methods used to fabricate semiconductor devices, and more specifically to a method used to form a gate insulator layer.
(2) Description of Prior Art
The continuing trend to thinner gate insulator layers, to improve device performance as well as to reduce device operating voltages, presents potential yield and reliability concerns with the use of the ultra-thin insulator layers. Increased leakage current and greater risk of dielectric breakdown are two of the concerns encountered when employing ultra-thin insulator layers as the gate dielectric for metal oxide semiconductor field effect transistor (MOSFET) devices. The use of nitrided silicon dioxide layers for use as gate insulator layers can reduce the risk of leakage current and dielectric breakdown when compared to non-nitrided, silicon dioxide counterparts. However the process used to convert a silicon dioxide layer to a nitrided silicon dioxide layer, a nitrogen plasma procedure, can result in device degradation if the silicon dioxide layer is thin. Nitrogen penetration through the thin silicon dioxide layer to the semiconductor substrate can result in decreased channel mobility. Therefore an effective method to create nitrided silicon dioxide layers, nitrogen plasma procedures, can be a risky option when applied to silicon dioxide layers less than about 20 Angstroms.
This invention will describe a method of forming a thin, less than 20 Angstroms, nitrided silicon dioxide layer, via use of a nitrogen plasma procedure, without the risk of deleterious semiconductor phenomena occurring as a result of nitrogen penetration. This is accomplished via formation of a thin, base silicon dioxide layer comprised with incorporated components which retard penetration of nitrogen to the silicon dioxide-semiconductor interface during the nitrogen plasma procedure. Prior art such as Sun et al, in U.S. Pat. No. 6,258,730 B1, Shue et al, in U.S. Pat. No. 6,197,701 B1, Okumo et al, in U.S. Pat. No. 6,110,842, Rodder et al, in U.S. Pat. No. 6,251,761, and Sun et al, in U.S. Pat. No. 5,880,040, describe processes for forming thin silicon dioxide, as well as for forming and nitrided silicon dioxide gate insulator layers. None of these prior arts however describe the novel process sequence now presented featuring specific conditions for preparation of a thin silicon dioxide layer allowing a subsequent plasma nitrogen procedure to be used to form the desirable nitrided silicon dioxide layer, without substrate and device degradation.
It is an object of this invention to form a thin gate insulator layer comprised with a top nitrided silicon dioxide component, on a bottom silicon dioxide component.
It is another object to this invention to form the top nitrided silicon dioxide component of the thin gate insulator layer, via a nitrogen plasma procedure performed to a thin silicon dioxide layer.
It is still another object of this invention to incorporate silicon oxynitride in the thin gate insulator layer, prior to the nitrogen plasma procedure to retard nitrogen from reaching the surface of semiconductor substrate.
In accordance with the present invention a process of forming a nitrided silicon oxide component for a thin silicon dioxide gate insulator layer, via a plasma nitrogen procedure, featuring formation of silicon oxynitride components in the pre-plasma nitrogen, thin silicon dioxide gate insulator layer to retard nitrogen radicals from reaching the surface of the semiconductor substrate, is described. A thin silicon dioxide layer is prepared for subsequent nitridization with incorporated silicon oxynitride components. A first method of forming the silicon dioxide layer with incorporated silicon oxynitride components features a thermal oxidation procedure performed using either nitrous oxide (N2O), nitric oxide (NO), or nitrous oxide/nitric oxide (N2O/NO), as reactants. A second method of forming the silicon dioxide layer features thermal oxidation of the base silicon dioxide layer followed by an anneal procedure, performed in an ambient comprised of either N2O, NO, or N2O/NO, again resulting in a silicon dioxide layer with incorporated silicon oxynitride components. A plasma nitrogen procedure is then applied to the silicon dioxide layer resulting in a thin silicon dioxide layer featuring a top portion of nitrided silicon dioxide. The presence of the silicon oxynitride component in the silicon dioxide layer prevented nitrogen radicals from penetrating the thin silicon dioxide layer during the plasma nitrogen procedure, thus preventing substrate damage and degraded device parameters.